1. Field of the Invention
This invention relates to improved chromium catalyst compositions and processes for utilizing same. The chromium catalysts are useful in the particle form polymerization process for the manufacture of high density polyolefin resins which exhibit high resistance to environmental stress cracking and which are useful for the production of blow molded articles. More specifically, the chromium catalyst compositions are mixed catalysts comprised of a first and a second chromium-containing supported catalyst component obtained using silica supports of different pore volumes and wherein one of the supported catalyst components also contains aluminum.
2. Description of the Prior Art
In the selection and production of resins for the manufacture of blow molded goods, more so than with any other fabrication method, a careful balance must be struck between the physical properties of the solid resin and the processing characteristics of the resin melt if efficient production of durable molded articles is to be achieved. While many resins have superior physical properties, they do not have acceptable rheological (viscoelastic) properties under conditions of flow and shear such as are encountered during blow molding. Conversely, other resins which exhibit satisfactory viscoelastic behavior are deficient in one or more essential physical characteristics. For this reason there is a continuing effort to develop resins which have an optimal balance of physical and rheological properties. This is especially true with polyethylene resins used for the manufacture of blow molded bottles.
Improved processability of polyolefin resins, i.e., improved flow properties and shear response, has been obtained by increasing the molecular weight distribution of the polymer. This can be accomplished by blending separately prepared polymers of different molecular weights or by utilizing catalyst systems capable of directly producing polymers having broadened molecular weight distributions. U.S. Pat. No. 4,025,707, for example, discloses the preparation of ethylene homopolymers and copolymers of broadened molecular weight distribution utilizing a mixed catalyst comprising several portions of the same or different chromium components and metal promoted variations thereof wherein each portion is activated at a different temperature. U.S. Pat. No. 4,560,733 discloses magnesium- and titanium-containing catalyst components for a similar purpose which are prepared by milling a blend of at least two different silica-containing components having different melt index potentials.
While it has been possible to improve processing characteristics in this manner, any processing advantage has heretofore been offset in large part by a corresponding decrease in one or more essential physical properties. For example, while the products obtained in accordance with U.S. Pat. No. 4,025,707 have good die swell characteristics and acceptable environmental stress cracking resistance and flow properties, polymer densities are too low to provide the necessary stiffness for blown bottles. On the other hand, polymers such as those produced using the catalysts of U.S. Pat. No. 4,560,733 have sufficiently high densities (0.960 and higher) but typically are deficient in their resistance to environmental stress cracking.
In addition to having acceptable processing characteristics, the resin must also have sufficiently high density and high resistance to environmental stress cracking in order to produce useful blown bottles. A density of at least 0.957 is necessary to obtain the high degree of stiffness required by molders. It is considered even more desirable for the resin to have a density of 0.958 to 0.961. Stiffness imparts strength to bottles and also makes thinner wall constructions possible. More units per pound of resin can thus be obtained which represents an economic advantage to the processor. The resin must also exhibit resistance to cracking under environmental stress, that is, while being stretched or bent in several directions at once.
While stiffness increases with density as noted above, an inverse relationship exists between density and resistance to environmental stress cracking, i.e., as density is increased, resistance to environmental stress cracking is reduced. These relationships assume that the melt index of the resins are the same or essentially the same. A balance must therefore be struck between the density, i.e., stiffness, of the resin and the resistance of the resin to environmental stress cracking.
It would be advantageous if polyethylene resins having an optimal balance of rheological and physical properties could be produced. It would be even more advantageous if high density resins having high resistance to stress cracking and which are useful for the manufacture of blown bottles could be obtained utilizing mixtures of known catalyst components. These and other advantages are realized using the mixed catalyst composition of the present invention which are described in detail to follow.